(I'm a graphic artist so I am biased….)

Hentai is just cartoons/illustrations/anime/comics….other than voiceover work, no actual human beings are involved. Just drawings. I think that any art is protected under the first amendment, I've never heard of illustrated images being "illegal".

Photography is treated differently because actual people are involved. So unless you are amazing at photoshopping adults into kids, then photo porn of the underage IS illegal.

As long as your BF has a firm grasp of reality vs fantasy I think you are fine. I don't like the underage/rape/violence/tentacle sex aspect of Hentai either but as long as he has a healthy life in general, I don't see a problem with a kink.